Shoe for a tubular element in a wellbore

ABSTRACT

A well casing or liner shoe ( 10 ) comprises a partially of fully openable central fluid channel ( 13 ) which is connected to at least one small width nozzle ( 46 ) and an additional large width fluid outlet ( 49 ), which is closed by a thin walled closure device arranged to open at a selected fluid overpressure in the fluid channel ( 13 ), and which has a flow area larger than each nozzle ( 46 ) to permit fluid circulation when the nozzles ( 46 ) are clogged and clearance of clogging debris through the additional large width additional fluid outlet ( 49 ).

The present invention relates to a shoe for a tubular element in aborehole extending into an earth formation.

Wellbores for the production of hydrocarbon fluid generally are providedwith steel casings and/or liners to provide stability to the wellborewall and to prevent undesired flow of fluid between the wellbore and thesurrounding earth formation. A casing generally extends from surfaceinto the wellbore, whereas a liner may extend only a lower portion ofthe wellbore. However in the present description the terms “casing” and“liner” are used interchangeably and without such intended difference.

In a conventional wellbore, the wellbore is drilled in sections wherebyeach section is drilled using a drill string that has to be lowered intothe wellbore through a previously installed casing. In view thereof thewellbore and the subsequent casing sections decrease in diameter withdepth. The production zone of the wellbore therefore has a relativelysmall diameter in comparison to the upper portion of the wellbore. Inview thereof it has been proposed to drill a “mono diameter” wellborewhereby the casing or liner to be installed is radially expanded in thewellbore after lowering to the required depth. Subsequent wellboresections may therefore be drilled at a diameter larger than in theconventional wellbore. If each casing section is expanded to the samediameter as the previous section, the wellbore diameter may remainsubstantially constant with depth.

During installation of the casing into the wellbore, either conventionalor mono diameter, the shoe at the lower end of the casing may encounterobstructions such as ledges, rock particles and debris. To overcome suchobstructions it has already been proposed to provide the casing shoewith reaming members and ports for pumping fluid into the wellbore.

U.S. Pat. No 6,401,820 B1 discloses a liner shoe provided with jettingports provided at an eccentric nose portion of the shoe.

Such jetting ports may become clogged, for example during pumping ofcement into the wellbore, thereby potentially leading to damage to theshoe and/or to unsuccessful cementation of the casing in the wellbore.

US patent application US2010/0252331 discloses a reamer shoe that isable to drill through modest obstructions within a previously drilledborehole and which is provided with nozzles (25) formed by apertures andfrangible regions that can be breached to form additional apertures (28)that may be used to provide fluid communication between the interior andexterior of the reamer shoe when the nozzles (25) are plugged. Adisadvantage of this known reamer shoe is that the frangible regions donot have larger flow areas than the nozzles, so that when the nozzles(25) are clogged by large size debris, also the additional apertures(28) will be clogged by such large size debris.

There is a need for an improved shoe for a tubular element in a boreholeextending into an earth formation, which overcomes the drawbacks of theprior art.

In accordance with the invention there is provided a shoe for a tubularelement in a borehole extending into an earth formation, the shoecomprising a body adapted to be connected to the lower end of thetubular element, the body being provided with:

-   -   a fluid channel;    -   at least one nozzle for jetting fluid into the borehole, each        nozzle being in fluid communication with the fluid channel; and    -   an additional fluid outlet for pumping fluid from the fluid        channel into the borehole, the fluid outlet being closed by a        closure device arranged to open at a selected fluid overpressure        in the fluid channel relative to a fluid pressure in the        borehole;    -   characterized in that the additional fluid outlet has a flow        area larger than each nozzle.

In this manner it is achieved that, in case one or more of the nozzlesbecome clogged, the fluid pressure in the fluid channel increases aspumping proceeds until the pressure reaches the selected overpressure atwhich the closure device opens and debris that plugged the nozzles iscleared via the additional fluid outlet. Thereby the fluid outletbecomes available for discharging fluid from the fluid channel into theborehole. Thus the closure device forms a safety system that preventsdamage to the shoe due to the pressure rating of the shoe beingexceeded. Moreover, if such clogging occurs during pumping cement intothe wellbore, the cementation procedure may proceed by pumping cementvia the fluid outlet into the borehole once the closure device hasopened.

Suitably the closure device includes a section of reduced wall thicknessof the body, which section is arranged to shear off from the body uponthe fluid pressure in the fluid channel reaching the selectedoverpressure. For example, the section of reduced wall thickness maycomprise a burst plate.

The fluid outlet may have a flow area of, for example, between about 2to 3 square inch (12 to 20 cm²). The total flow area of said at leastone nozzle may be less than 1 square inch (6.5 cm²).

Advantageously the fluid channel may be provided with a seat forreceiving a plug for closing the fluid channel. Further, the fluidchannel may be provided with locking means for locking the plug in thefluid channel. For example, the locking means may comprise a recessformed in a selected one of the fluid channel and the plug, and theother of the fluid channel and the plug may be provided with a lockingmember arranged to snap into the recess upon the plug landing on theseat. In one embodiment the recess comprises an annular groove formed inthe fluid channel, and the locking member comprises a lock ring providedto the plug.

To promote proper seating of the plug into the fluid channel, the bodymay be provided with a chamber arranged to receive debris from the fluidchannel, wherein the plug is adapted to move the debris from the fluidchannel into the chamber.

Suitably the body includes a flange provided with at least one radiallyextending rib, each rib being arranged to prevent rotation of the flangeduring drilling-out of the shoe in the borehole.

Also, the body may include a reamer for reaming the borehole duringlowering of the tubular element into the borehole. Each nozzle may bepositioned at the reamer.

To prevent rotation of the reamer with the drill bit or milling toolduring drilling-out the shoe, suitably the reamer has a nose sectionarranged eccentrically relative to a central longitudinal axis of thetubular element when the shoe is connected to the tubular element.Furthermore, the reamer may be provided with at least one cutter bladefor cutting rock particles during reaming of the borehole, each cutterblade being arranged to prevent rotation of the reamer duringdrilling-out the shoe in the borehole.

The tubular element may be, for example, a casing or a liner adapted tobe arranged in the borehole. Alternatively the shoe may be included in abottom plug arranged in an expandable casing or liner below an expanderfor expanding the casing or liner in the borehole.

The invention will be described hereinafter in more detail and by way ofexample, with reference to the accompanying schematic drawings in which:

FIG. 1 schematically shows an assembly including a first embodiment ofthe shoe of the invention;

FIG. 2 schematically shows the assembly after pumping cement into thewellbore;

FIG. 3 schematically shows the assembly during expansion of the cladelement;

FIG. 4 schematically shows the assembly after the bottom plug has beenset;

FIG. 5 schematically shows the assembly during drilling-out of thebottom plug;

FIG. 6 schematically shows a modified bottom plug for use in theassembly; and

FIGS. 7a-d schematically show an assembly including a second embodimentof the shoe of the invention.

In the description and the figures, like reference numerals relate tolike components.

FIG. 1 shows an assembly 1 for expanding a tubular element 2 in awellbore 3 extending into an earth formation 4. The assembly 1 comprisesa primary expander 6 connected to an expansion mandrel 8 suspended inthe wellbore 3 on a drill string (not shown) that normally may be usedfor drilling of the wellbore. The primary expander 6 has a cylindricalupper portion 6 a of diameter substantially equal to the inner diameterof the unexpanded tubular element 2 and a conical lower portion 6 b ofdiametrical size adapted to expand the tubular element 2 to the desireddiameter to form a liner in the wellbore 3. The tubular element 2 issuspended on the primary expander 6 whereby the cylindrical portion 6 athereof extends into the lower end of the tubular element 2.

The assembly 1 furthermore comprises a bottom plug 10 arranged below theprimary expander 6 and connected to a plug mandrel 12 in releasablemanner, the plug mandrel being fixedly connected to the lower end of theexpansion mandrel 8. The plug mandrel 12, the expansion mandrel 8 andthe drill string have a common fluid channel 13 for fluid pumped fromsurface to the bottom plug 10. The bottom plug 10 comprises a flange 14having a recess 16 into which a lower end part 18 of the plug mandrel 12fits. The recess 16 and lower end part 18 have complementary hexagonalshapes so as to allow torque to be transmitted between the plug mandrel12 and the bottom plug 10, however any other suitable shape may beselected to allow torque to be transmitted. A radially expandabletubular clad element 20 is fixedly connected to the flange 14 andextends coaxially around the plug mandrel 12. A secondary expander 22 isarranged inside the clad element 20, the secondary expander having acylindrical upper portion 22 a of diameter substantially equal to theinner diameter of the unexpanded clad element 20 and a conical lowerportion 22 b of maximum diameter adapted to expand the clad element 20against the inner surface of tubular element 2 after radial expansionthereof. The clad element 20 has a launcher section in the form of thinwalled lower section 24 with an oversized inner diameter to accommodatethe conical lower portion 22 b of the secondary expander. The cladelement further includes a lower anchoring section 26, an upperanchoring section 28 axially spaced from the lower anchoring section,and a sealing section 30 located between the lower and upper anchoringsections 26, 28. Each anchoring section 26, 28 is at the outer surfaceprovided with a coating of friction material, for example a coatingincluding carbide particles embedded in a substrate that is metallicallybonded to the outer surface by means of laser welding. The sealingsection 30 is at the outer surface provided with annular seals 34.

The plug mandrel 12 extends through a central bore 36 of the secondaryexpander 22 in a manner allowing the secondary expander 22 to slide inaxial direction along the plug mandrel 12. The plug mandrel 12 isprovided with flow ports 38 fluidly connecting the fluid channel 13 witha fluid chamber 40 formed between the large diameter end of thesecondary expander 22 and the flange 14. Initially the axial size of thefluid chamber 40 is very small but increases during expansion of theclad element 20 as will be explained hereinafter. The upper end of theclad element 20 is covered by a removable debris cap 42 having a centralbore 44 through which the plug mandrel 12 extends in a manner allowingthe debris cap 42 to slide in axial direction along the plug mandrel 12.The debris cap 42 serves to prevent debris entering the clad element 20prior to radial expansion thereof. Further, the bottom plug 10 isprovided with a reamer 45 having outlet openings in the form of nozzles46 in fluid communication with the fluid channel 13 via an internalchamber 47 of the reamer and a bore 48 in the flange 14. The chamber 47has a wall section of reduced thickness in the form of burst plate 49that is adapted to shear off at a selected fluid overpressure in thechamber 47 relative to a fluid pressure in the borehole i.e. exterior ofthe bottom plug 10. When sheared off, the burst plate 47 leaves a fluidoutlet from the chamber with a flow area larger than the flow area ofeach nozzle 46. The bore 48 has a seat 50 for receiving a trailing plug52 (FIG. 2) to close the bore.

FIG. 2 shows the assembly 1 whereby a fluidic cement column 53 surroundsthe tubular element 2 and the assembly 1. The trailing plug 52 isreceived on the seat of the bore 48 and thereby closes the bore.

FIG. 3 shows the assembly 1 after a lower portion 54 of the tubularelement 2 has been expanded by the primary expander 6, whereby thebottom plug 10 is positioned in the expanded lower portion 54 and theclad element 20 is partly expanded against the inner surface of theexpanded lower portion 54. A volume of hydraulic fluid 56, such asspacer fluid or drilling fluid, has been pumped into the fluid chamber40 via the fluid channel 13 and flow ports 38.

FIG. 4 shows the assembly 1 after the clad element 20 has been fullyexpanded against the inner surface of the expanded lower portion 54 ofthe tubular element 2, whereby the plug mandrel 12 is released from theflange 14. The secondary expander 22 and the debris cap 42 maintainpositioned at the plug mandrel.

FIG. 5 shows the assembly 1 after tubular element 2 has been fullyexpanded, and the expansion mandrel 8 and the plug mandrel 12 togetherwith the secondary expander 22 and the debris cap 42 have been removedfrom the wellbore 3. A drill string 58 with a polycrystalline diamondcompact (PDC) bit 60 is lowered into the expanded tubular element 2 todrill out the remainder of the bottom plug 10. Instead of the PDC bit60, a dedicated milling tool may be applied to drill out the remainderof the bottom plug.

FIG. 6 shows a modified bottom plug 64 which is substantially similar tothe bottom plug 10 except regarding the following. The reamer 45 has anose section 66 arranged eccentrically relative to a centrallongitudinal axis of the plug mandrel 12. Furthermore, the modifiedbottom plug 64 is provided with an activation sleeve 68 positioned inthe bore 48 to temporarily close the flow ports 38. The activationsleeve 68 is locked in place by suitable shear pins (not shown) and isadapted to slide axially downward through the bore 48 when the shearpins are broken whereby the flow ports 38 are freed. The seat 50 for thetrailing plug 52 is provided in the activation sleeve 68 rather than inthe bore 48. Furthermore, the modified bottom plug 64 is provided with aprotective sleeve 70 extending around the sealing section 30 and theanchoring sections 26, 28 of the clad element 20. The sleeve 70 isfixedly connected to the debris cap 42, the latter having a cylindricalpart 42 a that extends into the clad element 20 and abuts against thesecondary expander 22. Reamer 45, flange 14 and clad element 20 areinterconnected by a crossover sub 71.

FIGS. 7a to 7d show an assembly 80 using a second embodiment of the shoeof the invention, whereby FIG. 7a shows a side view and FIGS. 7b-d showa longitudinal section of the assembly. The assembly 80 includes a shoe82 connected to the lower end of a wellbore casing (or liner) 84. Theshoe 82 is provided with a reamer 86 having an eccentric nose section 87relative to a central longitudinal axis of the casing 84. The reamer 86is provided with nozzles in the form of flow ports 88 to enable drillingfluid to be circulated during running into a wellbore. The size of theflow ports 88 is such that the total flow area of the flow ports 88 issimilar to, for example, the flow area of a conventional PDC drill biti.e. typically less than 1 square inch so as to provide the necessaryjetting action for hole cleaning purposes. The flow ports 88 arearranged such that an optimum flow distribution and scavenging of thewellbore by jet action is obtained. The inner surfaces of the flow portsmay be provided with a hard facing to mitigate erosion duringcirculation.

The reamer 86 is at its outer surface provided with a plurality ofcutter blades 90 having abrasion resistant cutting elements 92 e.g.carbides that may be brazed on the outer surface of the reamer. Thereamer 86 is connected to the casing 84 by means of a crossover sub 94which forms the transition between the casing 84 and a flange 96 of theshoe 82. The flange 96 is provided with a profiled bore 98 having a seat99 for receiving a trailing plug 102 (FIG. 7d ) used during pumpingcement into the wellbore. The bore 98 is in fluid communication with theflow ports 88 via a chamber 103 formed in the shoe. A groove 104 isprovided in the bore 98 to enable the trailing plug 102 to be locked inplace once seated (FIG. 7d ). The flange 96 may optionally be providedwith radial ribs (not shown) that are surrounded by cement aftercementation to prevent rotation of the flange 96 during drilling out theshoe from the wellbore. Further, the flange 96 may optionally beprovided with arms (not shown) connected to the reamer 86 to providemechanical support to the reamer. The reamer 86 has a section of reducedwall thickness in the form of burst plate 108 that is adapted to shearoff from the reamer at a selected fluid overpressure in the chamber 103relative to a fluid pressure in the borehole i.e. exterior of the shoe82. When sheared off, the burst plate 108 leaves an outlet opening 109of chamber 103 with a flow area larger than the flow area of each flowport 88 (FIG. 7c ).

Normal operation of the assembly 1 is as follows. The assembly 1 islowered into the wellbore 3 on drill string and may be rotated to reamsections of the wellbore 3 by reamer 45. Simultaneously drilling fluidmay be pumped into the wellbore via fluid channel 13, chamber 47 andnozzles 46. The pumped fluid assists in reaming the wellbore andtransports rock particles to surface. Once the assembly 1 has reachedtarget depth of the wellbore, the tubular element 2 is at its upper endanchored in the wellbore 3. Subsequently a volume of leading spacerfluid (not shown) is pumped into the wellbore via the fluid channel 13to clean the fluid channel from drilling fluid, followed by the fluidiccement column 53 and the trailing spacer fluid 56 (FIG. 3). Instead oftrailing spacer fluid, drilling fluid may be used. The leading spacerfluid and the fluidic cement 53 may be separated by a foam ball thatcrushes upon arriving in the bore 48 of the bottom plug 10 and isreleased through the outlet openings 46. The fluidic cement and thetrailing spacer fluid 56 are separated by the trailing plug 52 thatseats on the seat 50 upon arriving in the bore 48. Thus, at this stagethe trailing spacer fluid 56 is present in the fluid channel 13, and thecement column surrounds the bottom plug 10 and the tubular element 2.The trailing plug 52 closes the bore 48 and thereby seals the fluidchannel 13 from the annular space around the assembly 1 in the wellbore3. The primary expander 6 abuts against the lower end of the tubularelement 2 therefore fluidic cement cannot enter the tubular element 2(FIG. 2).

In the event that one or more of the nozzles 46 become clogged beforethe fluidic cement column 53 has been fully pumped into the wellbore,the fluid pressure in chamber 47 increases as pumping continues untilthe burst plate 49 shears off upon the fluid pressure reaching theselected overpressure. Thereby, the relatively large flow area in thewall section of the former burst plate becomes available for pumpingcement into the wellbore. In this manner the burst plate 49 forms acontingency device that prevents a catastrophic situation whereby thecementing procedure cannot be completed successfully.

The burst plate 49 also protects the reamer 45 against pressureshockwaves that may occur in the fluid channel 13, for example duringsetting of a liner hanger or the activation of the expansion system.Such shockwaves may have amplitudes up to 3500 psi and will almostentirely reflect against the reamer 45 thereby doubling in amplitude.The burst plate 49 shears off at such high pressure peaks and therebyprotects the reamer 45 against damage or failure.

Furthermore, debris that may be present in the bore 48 is pushed intothe chamber 47 by the trailing plug 52 as it moves into the bore 48. Inthis manner proper seating of the trailing plug 52 in the bore is nothampered by such debris.

After seating of the trailing plug 52 in the bore 48, the primaryexpander 6 is pulled into the tubular element 2 by pulling the drillstring whereby the lower portion 54 of the tubular element 2 is expanded(FIG. 3). Expansion is proceeded until the bottom 10 plug is fullyinside the expanded lower portion 54. While maintaining the drill stringunder tension, fluid pressure is applied in the fluid channel 13 so thatthe trailing spacer fluid 56 flows via the flow ports 38 of the plugmandrel 12 into the fluid chamber 40. The secondary expander 22 therebyslides along the plug mandrel 12 away from the flange 14 and graduallyexpands the clad element 20 against the expanded lower portion 54 of thetubular element 2. The lower anchoring section 26 first engages theexpanded lower portion 54, followed by the sealing section 30 andsubsequently the upper anchoring section 28. Upon the sealing section 30engaging the expanded lower portion 54, the tubular element 2 issimultaneously further expanded with the primary expander 6 to maintainvolume balance in the expanded section of the tubular element 2 betweenthe bottom plug 10 and the primary expander 6.

Once the clad element 20 is fully expanded against the expanded tubularelement 2, the secondary expander 22 moves out of the clad element andthereby pushes the debris cap 42 off the clad element 20. The interiorof the expanded clad element 20 is then filled with trailing spacerfluid that may be contaminated with cement. In a subsequent step theremainder of the tubular element 2 is expanded with the primary expander6 whereby the secondary expander 22 and the debris cap 42 are carriedout of the wellbore 3 on the plug mandrel 12 (FIG. 4). After the bottomplug 10 has been set in the expanded lower portion 54 of the tubularelement, fluid pressure can be applied below the primary expander 6 viathe fluid channel 13 to provide additional upward force to the primaryexpander 6 (hydraulic assist). Alternatively, the entire expansion forcerequired to expand the tubular element 2 may be provided by such fluidpressure i.e. without applying tensile force to the drill string.

The design functionalities of the upper and lower anchoring sections 26,28 and the sealing section 30 are as follows. When the fluid pressure inthe interior space of the fully expanded clad element 20 is higher thanthe fluid pressure below the bottom plug 10, the clad element issubjected to balloon deformation whereby the lower anchoring section 26becomes firmly pressed against the expanded tubular element 2.Conversely, when the fluid pressure below the bottom plug 10 is higherthan the fluid pressure in the interior space of the fully expanded cladelement 20, for example due to swab pressure below the primary expander6 during expansion of the tubular element 2, the clad element issubjected to balloon deformation whereby the upper anchoring section 28becomes firmly pressed against the expanded tubular element 2.

After the cement has fully cured, the bottom plug 10 is drilled out withthe PDC bit 60 or milling tool on drill string 58 whereby the bottomplug is supported by the cement 53 surrounding it (FIG. 5).

In a variation of normal use, the cement 53 is pumped into the wellboreafter the lower portion 54 of the tubular element has been expanded andthe bottom plug 10 has been pulled into the expanded lower portion 54.This approach may be followed if there is a risk that the secondaryexpander 22 is activated before the bottom plug 1 is inside the lowerportion 54 of the tubular element, e.g. due to pressure waves in thefluid channel 13 propagating into the fluid chamber 40 during pumping ofcement into the wellbore. However since in the alternative method thereis reduced annular space between the expanded lower portion 54 of thetubular element and the wellbore wall, the pressure drop required topump the cement at a certain rate through the annular space increases,which may lead to an increased risk of formation fracturing in criticalpressure regimes.

Stabilization of the PDC bit or milling tool 60 during drilling-out ofthe bottom plug 10 may be optimized as follows. In the methods describedabove the clad element 20 is hydraulically expanded with the trailingspacer fluid 56 as a pressure medium. Consequently after completion ofthe expansion process the interior of the clad element 20 is filled withtrailing spacer fluid that may be contaminated with some cement. Inorder to optimize stabilization of the PDC bit or milling tool 60 duringdrilling-out of the bottom plug 10 an additional volume of cement may bepumped behind the trailing plug 52 to expand the clad element 20. Atrailing foam ball (not shown) may be pumped behind the cement,optionally followed by trailing spacer fluid. After the trailing plug 52has seated in the bore 48, the installation process is continued asdescribed above whereby the pressure medium used for the expansion ofthe clad element 20 is cement rather than trailing spacer fluid ordrilling fluid. During expansion of the tubular element 2 the trailingfoam ball is pumped out of the plug mandrel 12 into the wellbore. Thus,after curing of the cement the bottom plug 10 is surrounded by curedcement, optionally with excess cured cement above the clad element 20 tomitigate the risk of damage to the PDC bit or milling tool 60 upontagging the bottom plug 10 and to provide optimum conditions fordrilling-out of the bottom plug 10.

In addition to the above, the risk of damage to the cutters of the PDCbit or milling tool 60 when tagging the top of the clad element 20 canbe further mitigated by connecting a short pipe section (not shown) of asoft metal, for example copper, to the top of the clad element 20. Thepipe section is subjected to plastic deformation due to loading by thePDC cutters thereby limiting the peak contact load and thus the risk ofimpact damage to the PDC cutters.

Normal operation of the assembly 1 when provided with the modifiedbottom plug (FIG. 6) is substantially similar to normal operationdescribed above. In addition the eccentric nose section 66 of the reamer45 helps in preventing rotation of the reamer during drilling out thebottom plug 10 with the PDC bit 60 or the milling tool. The activationsleeve 68 prevents unintentional expansion of the clad element 20 by thesecondary expander 22 due to fluid pressure peaks in the fluid channel13 before the trailing plug 52 has landed in the activation sleeve. Asthe trailing plug 52 lands into the activation sleeve 68, the trailingplug pushes the activation sleeve downward whereby the shear pins 69 arebroken and the flow ports 38 are freed. Furthermore, the protectivesleeve 70 protects the sealing section 30 and the anchoring sections 26,28 before expansion of the clad element 20. During expansion of the cladelement 20, the protective sleeve 70 moves in axial direction away fromthe clad element 20 together with the debris cap 42. In this manneroptimum protection is provided to the sealing section 30 and theanchoring sections 26, 28 which become exposed only just before thesecondary expander expands these sections.

Normal operation of the assembly 80 (FIGS. 7a-d ) is as follows. Theshoe 82 is connected to the lower end of the casing 84 and the assembly80 is run into a wellbore (not shown). When an obstruction isencountered in the wellbore, e.g. ledges in a deviated hole, theassembly 80 may be rotated while moving down to enable the reamer 86 toovercome the obstruction. If debris has accumulated in the wellbore dueto, for example, downward movement of the assembly in a deviatedwellbore, drilling fluid may be circulated simultaneously via the flowports 88 to remove such debris by jetting action. In case of mechanicalhole stability problems, rock cavings that cannot be removed by jettingonly may obstruct lowering of the assembly 80. In such case the cutterblades 90 cut the cavings into jettable chunks which are transported tosurface by the circulating drilling fluid. Once the target depth of theopen hole has been reached, a volume of cement is pumped into thewellbore via the bore 98, the chamber 103 and the flow ports 88. Thecement fills up the annulus between the casing and the wellbore wall.Following the cement, the trailing plug 102 is pumped onto the seat 99of the flange 96 (FIG. 7d ). An elastomer seal (not shown) at the outersurface of the trailing plug seals the trailing plug relative to thebore 98. A lock ring 110 of the trailing plug snaps into the groove 104so as to prevent the trailing plug from being pushed back by U-tubingpressure of the cement. Any debris that may be present in the bore 98 ispushed into the chamber 103 by the trailing plug 102 as the latter landsinto the bore 98.

The relatively small size of the flow ports 88 (required to provide thenecessary jetting capability during reaming) involves the risk ofclogging of the flow ports 88 during cementation. In case of suchclogging (FIG. 7c ) the fluid pressure in the reamer 86 will increaseuntil the pressure rating of the burst plate 108 is exceeded so that theburst plate shears off. Thereby a new flow port 112 with a flow area ofsome 2-3 square inches is provided which is typical for conventionalcementing shoes. This enables the cementing operation to be completedsuccessfully by pumping the remainder of the cement via the new flowport 112 into the annulus.

Once the cement is cured the shoe 82 is drilled out preferable using aPDC drill bit. The components of the shoe 82 are bonded to the curedcement and therefore are locked in place during drilling out. Forexample, the ribs (if present) and the cutter blades 90 assist in suchlocking of the components. Further, the eccentric nose section 87 of thereamer 86 prevents rotation of the reamer with the drill bit. Allcomponents to be drilled out are suitably made from easily drillablematerial e.g. cast iron or aluminium, and the cutting elements on thecutting blades are of small size and are embedded in a relatively softsubstrate such as brazing material. In this manner the hard cuttingelements can easily be broken out of the substrate and discharged fromthe wellbore by the drilling fluid.

The present invention is not limited to the above-described embodimentsthereof, wherein various modifications are conceivable within the scopeof the appended claims. For instance, features of respective embodimentsmay be combined.

1. A shoe for a tubular element in a borehole extending into an earthformation, the shoe comprising: a body adapted to be connected to adownhole end of the tubular element, the body being provided with: afluid channel; at least one nozzle for jetting fluid into the borehole,each nozzle being in fluid communication with the fluid channel and anadditional fluid outlet for pumping fluid from the fluid channel intothe borehole, the fluid outlet being closed by a closure device arrangedto open at a selected fluid overpressure in the fluid channel relativeto a fluid pressure in the borehole; characterized in that theadditional fluid outlet has a flow area larger than each nozzle.
 2. Theshoe of claim 1, wherein the closure device includes a section ofreduced wall thickness of the body, which section is arranged to shearoff from the body upon the fluid pressure in the fluid channel reachingthe selected overpressure.
 3. The shoe of claim 2, wherein the sectionof reduced wall thickness comprises a burst plate.
 4. The shoe of claim1, wherein the fluid outlet has a flow area between about 2 to 3 squareinch (12 to 20 cm²).
 5. The shoe of claim 1, wherein the total flow areaof said at least one nozzle is less than 1 square inch (6.5 cm²).
 6. Theshoe of claim 1, wherein the fluid channel is provided with a seat forreceiving a plug for closing the fluid channel.
 7. The shoe of claim 6,wherein the fluid channel is provided with locking means for locking theplug in the fluid channel.
 8. The shoe of claim 7, wherein the lockingmeans comprises a recess formed in a selected one of the fluid channeland the plug, and wherein the other of the fluid channel and the plug isprovided with a locking member arranged to snap into the recess upon theplug landing on the seat.
 9. The shoe of claim 8, wherein the recesscomprises an annular groove formed in the fluid channel, and wherein thelocking member comprises a lock ring provided to the plug.
 10. The shoeof claim 6, wherein the body is provided with a chamber arranged toreceive debris from the fluid channel, and wherein the plug is adaptedto move the debris from the fluid channel into the chamber.
 11. The shoeof claim 1, wherein the body includes a flange provided with at leastone radially extending rib, each rib being arranged to prevent rotationof the flange during drilling-out of the shoe in the borehole.
 12. Theshoe of claim 1, wherein the body includes a reamer for reaming theborehole during lowering of the tubular element into the borehole. 13.The shoe of claim 12, wherein each nozzle is positioned at the reamer.14. The shoe of claim 12, wherein the reamer has a nose section arrangedeccentrically relative to a central longitudinal axis of the tubularelement when the shoe is connected to the tubular element.
 15. The shoeof claim 12, wherein the reamer is provided with at least one cutterblade for cutting rock particles during reaming of the borehole, eachcutter blade being arranged to prevent rotation of the reamer duringdrilling-out of the shoe in the borehole.
 16. The shoe of claim 1,wherein the closure device comprises a section of reduced wall thicknessof the body.
 17. The shoe of claim 16, wherein the section is arrangedto shear off from the body upon fluid pressure in the fluid channelreducing the selected fluid overpressure.
 18. The shoe of claim 16,wherein the section comprises a burst plate.
 19. The shoe of claim 16,wherein the section of reduced wall thickness is integral part of thebody.